The present invention relates to a magnetic recording disk constituted by a flexible magnetic sheet having a magnetic layer that is stretched across a disk-shaped substrate having a central bore and a broad annular recess in the surface. More particularly, the invention relates to a magnetic disk having a more rigid substrate.
Magnetic disks, such as floppy disks and rigid magnetic disks, are used in a broad range of applications including computers, office automation (OA) equipment and electronic devices. Floppy disks are manufactured by stamping out disks of a flexible magnetic sheet having a magnetic layer on a flexible nonmagnetic support, and placing the stamped-out disks within a comparatively soft jacket or a highly rigid case for use as external memories. Rigid magnetic disks, which commonly are used as built-in memories within equipment, are manufactured by forming a magnetic layer on a rigid substrate made of a less flexible material such as aluminum.
One of the requirements of modern floppy disks and rigid magnetic disks is increased recording density and capacity. This need can be met in one way by increasing the density of recording tracks on the surface of the disks.
Floppy disks which use flexible supports made of synthetic resins exhibit particularly high endurance when rotated in sliding contact with a magnetic head. Further, the magnetic layer used in floppy disks is highly amenable to surface treatments, so the center-line-average roughness (Ra) of the recording surface of the magnetic layer can be reduced to a very small value. Therefore, so far as the surface property of the magnetic layer is concerned, floppy disks theoretically should be compatible with the requirement for increased recording density. However, if the recording capacity and, hence, the recording density is to be increased by forming more tracks along the periphery, the support, which is made of a synthetic material, will experience substantial expansion or contraction because of temperature changes. Such dimensional change often has caused tracking errors, or positional offsets from the magnetic head.
On the other hand, rigid magnetic disks which have a magnetic layer formed on a substrate that is comparatively rigid and has a low thermal expansion coefficient are less prone to tracking errors from expansion or contraction under varying temperatures. Hence, the aforementioned problem of floppy disks associated with tracking errors is alleviated considerably with rigid magnetic disks.
In the manufacture of rigid magnetic disks, the surface of a substrate typically made of aluminum is ground and polished to such a level that the spacing loss between the magnetic head and the magnetic disk is made sufficiently small to permit high-density recording. Only thereafter is a magnetic layer formed on the substrate. To perform magnetic recording and reproduction at high density, the surface of the magnetic disk preferably is as smooth as possible. However, with conventional techniques of surface treatment, it has been difficult to achieve a substrate surface that has a center-line-average roughness (Ra) below 0.1 .mu.m. This has limited previous attempts to meet the requirement of rigid magnetic disks for higher recording density by improving the substrate surfaces.
Further, the substrate used as the support of the magnetic layer in rigid magnetic disks is not flexible. Thus, in the process of manufacturing rigid magnetic disks, a convenient method cannot be adopted in which the support wound onto a roll is unwound toward the subsequent step of forming a magnetic layer on the support. As a result, the manufacturing process has been inconvenient in that a magnetic layer cannot be continuously formed on a long support. Another problem associated with the lack of flexibility in the support is that, if the magnetic head accidentally contacts the magnetic layer during recording or reproduction (in either mode, such contact should not occur), a great impact will occur readily, potentially destroying the magnetic layer. This problem is particularly serious when one attempts to achieve further improvement in the density of magnetic recording and reproduction since it is required to reduce the spacing loss between the magnetic head and the magnetic layer, making the frequency of damage to the magnetic layer even higher.
With a view to solving the aforementioned problems, a new type of magnetic disk has been proposed. Called a stretched surface recording (SSR) disk, this new version of magnetic disk is constituted by a rigid substrate having a broad annular recess in the surface and a flexible magnetic sheet having a magnetic layer on the surface of a flexible nonmagnetic support that is stretched across said rigid substrate in such a way that it covers the annular recess while forming a space under the magnetic sheet as defined by the recess.
The recording surface of this new type of magnetic disk is flexible (i.e., it is formed as a flexible sheet), so that, even if the magnetic head accidentally contacts the recording surface of the magnetic layer, and even if high-density recording or reproduction is performed with the head being held in sliding contact with the magnetic layer (i.e., as if they were in mutual contact), the magnetic layer is less likely to be destroyed than would be the case for conventional rigid magnetic disks. Further, the advantageous features of floppy disks are retained, enabling the manufacture of magnetic disks having a good surface property and high endurance in rotation. More specifically, the annular recess in the surface of the rigid substrate provides a space under the magnetic sheet. Even if the magnetic sheet accidentally contacts the magnetic layer, the space permits the magnetic sheet to deform elastically, thereby exhibiting a cushioning effect to prevent the head from destroying the magnetic layer. In addition, the spacing loss between the magnetic layer and the magnetic head is reduced further to allow optimum sliding for high-density recording. Thus, the SSR disk may well be regarded as a magnetic disk that offers the inherent advantages of a floppy disk and a rigid magnetic disk.
General technical information on the SSR disk may be found in U.S. Pat. No. 4,573,097. As shown in FIG. 4, a magnetic layer 11 is formed by some suitable method on a flexible support 12 made of a synthetic resin. As shown in FIG. 3, the resulting magnetic sheet 2 is stretched across each side of a rigid substrate 3 having not only a central bore 9 but also an annular recess 8 that is formed in each of the top and bottom surfaces. The magnetic sheet 2 is stretched across the rigid substrate 3 in such a way as to cover the recesses 8.
The magnetic disk 1 in FIG. 3 is produced by a process that includes stamping out a predetermined shape of the magnetic sheet 2 and then bonding it onto a boss 6 and an annular upstanding rim 7 formed on either side of the rigid substrate 3, with the magnetic layer 11 facing upwardly. Thus, a space 5 is formed between the magnetic sheet 2 and the rigid substrate 3 that corresponds to the depth of the recess 8 on either side of the substrate 3. In bonding the magnetic sheet 2 onto the rigid substrate 3, it is necessary that the sheet 2 be stretched in such a way as to withstand compression as with the magnetic head. To meet this need, the magnetic sheet 2 usually is stretched with sufficient tension being applied to make it taut.
To maintain the tension applied to the magnetic sheet 2, the substrate 3 typically is made of strong metallic materials such as aluminum. However, metallic materials are very expensive and, furthermore, are not very easy to process. Under these circumstances, it may be proposed that polymeric materials that are not only low in material cost but which also permit efficient production of substrates be used as substitutes for aluminum.
However, in practice magnetic disks using substrates that are produced from polymeric materials are not rigid enough to insure prolonged maintenance of the tension applied to the magnetic sheet 2. Further, if the substrate is formed by injection molding, internal strains are likely to occur which will eventually lead to warpage or distortion of the substrate. During prolonged use, the tension applied to the magnetic sheet will cause a gradual increase in the deformation of the substrate. As a result, the magnetic disk will rotate erratically or unevenly, increasing the chance of deterioration in quality such as the inability to operate in close conformity to the magnetic head.